The present invention relates generally to multicylinder internal combustion engines of a variable displacement type in which a number of cylinders is deactivated in response to sensed engine load, and specificially it relates to a closed loop controlled electronic fuel injection of the variable displacement type in which separate exhaust systems and exhaust gas sensors are provided for separate cylinder groups.
Variable displacement internal combustion engines are known in the art to improve fuel economy by selectively shutting off fuel supply to several cylinders of the engine when reduced power output can operate the vehicle adequately. This variable displacement control is particularly advantageous for application to electronic fuel injection because the fuel injectors can be electrically disabled to cut off fuel without having the need for mechanical parts to shut off intake valves which would otherwise be required in carbureted engines. On the other hand, closed loop fuel control approach is known as an effective method for minimizing the harmful products HC, CO and NOx by maintaining air fuel ratio within a narrow range of catalytic conversion using a feedback signal obtained from an exhaust gas sensor. When closed loop controlled electronic fuel injection is operated in displacement mode where several cylinders are unfueled, the unfueled cylinders draw in air and exhaust the same through exhaust pipe where the exhaust gas sensor and catalytic converter are provided. Therefore, the gas sensor will generate a signal indicative of leaner mixtures than the mixture supplied to the working cylinders, and the air fuel ratio within the exhaust system will stray off the converter window where the catalyst simultaneously provides oxidation of unburned fuel and the reduction of nitrogen oxides. Thus, the closed loop fuel control cannot properly operate.